Inks that are utilized to print documents in color using a dot matrix format are subject to several demanding and often conflicting limitations. The ink must adhere to the medium without soaking into the medium and bleeding excessively lest the sharpness or resolution of the recorded image be reduced to an unacceptable level. As an example of the degree of sharpness required, current commercial applications require a resolution in the order of 240.times.400 (i.e., 240 dots per inch in the horizontal direction and 400 dots per inch in the vertical direction) for text printing. High quality graphic printing requires resolutions from 600 to 1000 dots or resolution elements per inch in each direction. Ink must also be resistant to abrasion as well as to smudging or inadvertant transfer or offsetting to adjacent materials. Additionally, since a particular printed document may pass through a variety of climates and office environments, the ink must remain stable at temperatures ranging from -20.degree. C. to +150.degree. C.
The restrictions are quite severe when applied to black ink, but are even more stringent when applied to color inks because even minor changes in color and hue are noticible to the human eye. In the case of inks that are to be applied by ink jet printing techniques, the problem is futher compounded by the requirement that the inks be compatible with the jetting requirements of the printer. Typically this application imposes tight constraints on the characteristics of the ink so that the ink can be expelled properly from the jet nozzle as relatively uniform droplets without excessive satelliting. Thus the physical properties of the ink such as viscosity and surface tension must be within certain ranges if the ink is to be jetted properly. Also, the ink must not clog the nozzle orifice and it must produce images of sufficient optical density to create a good quality printed record. Additionally, the ink must have a high rate of fixing to the recording medium and not wrinkle, curl or otherwise adversely affect the medium.
Satisfactory color printing by ink jet involves, then, the formation on the recording medium of a multiplicity of colored dots or spots of different color intensities, depending upon the color requirements of the various parts of the character or picture being printed on the medium. This wide color spectrum can be obtained by using three or four different color inks, either by an additive color mixing process or by a subtractive mixing process. In the former, red, green, blue and sometimes black ink drops are deposited on the medium side by side in a dot matrix. The different color dots are integrated in the observer's eye so that he percieves colors dependent upon the relevant numbers and/or sizes of the different color dots at each part of the printed character or picture.
In the subtractive process, the printer deposits ink drops of the primary subtractive colors, namely cyan, magenta and yellow, and perhaps also black, on the medium in superposition in a dot matrix arrangement so that each dot is composed of one or more layers, up to three or four, of printing inks having different color intensities depending upon the color requirements of the particular portion of the character or picture being printed. Each dot layer absorbs a portion of the spectrum of the ambient light illuminating the medium so that the viewer's eye senses the remainder of the light spectrum. Thus, when several ink layers, each of a different primary color, are present in a single dot in the matrix, several spectral portions are absorbed simultaneouslv so that a smaller part of the spectrum of incident light is reflected causing the viewer's eye to sense a mixed color produced by the subractive mixture of the primary colors. Thus, true color printing using the subtractive color mixing process depends upon the precision of the printing process as well as the optical properties of the different color printing inks. The color graphics industry prefers to print by subtractive color mixing because the resultant color print is brighter and more vivid than that resulting from additive color mixing, particularly when the power of the ambient light is relatively low.
Known commercial printing inks can be divided into three different types, namely water-base inks, oil-base inks and hot-melt or plastic-base inks. Presently, only the first two ink types are used in commercial jet printers. However, the quality of the printed copy produced by those inks is not as high as might be desired. The resultant interaction of the water or oil in the ink and the recording medium distorts the medium; it also results in the incident light being reflected diffusely from the color dots or resolution elements on the medium so that the observed printed colors are muted. Further, those inks have low surface tensions so that when deposited on the recording medium, they tend to form dots which are fairly irregular in shape. In short, color printing by ink jet has not gained wide acceptance because the quality of the color copy is not even good enough to meet the standards of the printing industry, much less of the color graphics industry.
Very recently, an ink jet printer has been developed which is able to print on a conventional recording medium such as ordinary paper using special hot-melt subtractive color inks. This printer and the inks used therein are disclosed in U.S. Pat. applications Ser. Nos. 668,000, filed Dec. 31, 1984; 748,768, filed June 25, 1985 and 749,681, filed June 24, 1985. All of these pending applications are owned by the assignee of the present application and their disclosures are incorporated by reference herein. Using the printer and inks described therein, one can produce by ink jet printing, color text and graphics which are superior in quality to the color copy made using conventional jet printers and inks. However, the resulting color print is still not bright and vivid enough to entirely satisfy the stringent requirements of the color graphics industry. In other words, the color records produced by the new ink jet printer and inks still do not match the quality of the records made using conventional offset printing techniques in terms of color brightness and optical clarity. Moreover, the quality of all these prior printed records deteriorates oven time due to the effects of moisture, oxidation and exposure to ultraviolet light from the sun and some fluorescent lights. Therefore, it would be desirable to improve the color printing resulting from the use and application of jet printers and inks, particularly the ones diclosed in said pending applications, so that the quality of color records produced by them compare favorably to the color records produced using known offset printing and photographic techniques.